When hydrocarbon producing wells are drilled, initial hydrocarbon production is usually attained by natural drive mechanisms (water drive, solution gas, or gas cap, e.g.) which force the hydrocarbons into the producing wellbores. If a hydrocarbon reservoir lacks sufficient pore pressure (as imparted by natural drive), to allow natural pressure-driven production, artificial lift methods (pump or gas lift, e.g.) are used to produce the hydrocarbon.
As a large part of the reservoir energy may be spent during the initial (or "primary" production, it is frequently necessary to use secondary hydrocarbon production methods to produce the large quantities of hydrocarbons remaining in the reservoir. Waterflooding is a widespread technique for recovering additional hydrocarbon and usually involves an entire oil or gas field. Water is injected through certain injection wells selected based on a desired flood pattern and on lithology and geological deposition of the pay interval. Displaced oil is then produced into producing wells in the field.
Advancements in secondary hydrocarbon producing technology has led to several improvements in waterflood techniques. For example, the viscosity of the injected water can be increased using certain polymer viscosifiers (such as polyacrylamides, polysaccharides, and biopolymers) to improve the "sweep efficiency" of the injected fluid. This results in greater displacement of hydrocarbons from the reservoir.
Ability to displace oil from all the producing intervals in a hydrocarbon reservoir is limited by the lithological stratification of the reservoir. That is, there are variations in permeability which allow the higher permeability zones to be swept with injected fluid first and leave a major part of the hydrocarbon saturation in the lower permeability intervals in place. Continued injection of flooding fluid results in "breakthrough" at the producing wells at the high permeability intervals which renders continued injection of the flooding medium uneconomical.
Profile control has been used to prevent or correct "breakthrough" at high permeability intervals. Profile control involves blocking off the higher permeability intervals in a mature flood so that the flooding media is diverted to lower permeability intervals. A gel treatment can be used to reduce the permeability of a higher interval or zone and thereby improve the sweep efficiency. The treatment must be selective, otherwise, it will not be effective and may even be damaging if underswept zones are plugged. To avoid this, a mechanical packer can be used to treat each strata separately but this procedure is expensive and rather tedious.
Another approach is to use polymer gels having selective penetration properties which will preferably enter the high permeability zones. However, such a gel is very rare. Chromium crosslinked xanthan gum is selective, but it is limited to reservoirs with a maximum temperature of 140.degree. F.
Another selective gel system is chromium crosslinked aminoresin stabilized xanthan gum disclosed in the U.S. Pat. No. 4,716,966. This patent is hereby incorporated by reference herein. This system is thermally stable up to 210.degree. F. No known gel systems are both selective and stable at temperatures higher than 120.degree. F. Resorcinol and formaldehyde are known to form brittle gels at a pH equal to or greater than 9 but such gels are brittle and lack selectivity. When the pH is about 9 or less a solid gel does not form. If a gel were to form it would be of poor quality.
Therefore, what is needed is a gel that can selectively enter pores in a formation's zone of greater permeability and subsequently reheal to form a shear and thermally stable gel where the temperature is greater than about 210.degree. F. in a pH environment of from about 3 to about 10.